This invention relates generally to ultrasonic imaging systems for diagnostic use or non-destructive testing, and more particularly to the real-time reduction of noise in such systems.
Ultrasonic imaging is being increasingly used in the medical community as a diagnostic technique for providing visual representations of tissue interfaces in a living subject. It is particularly useful for imaging organs which tend to shift or move, such as the heart. Echocardiographic images enable a physician to study the heart of a living patent non-invasively and to obtain important information about its condition.
The usefulness of medical ultrasound as a diagnostic technique, however, has been limited to some extent by the very poor signal-to-noise ratio in the resulting images. When ultrasound is injected in the body and encounters tissue, one of two interactions occurs. The first is reflection from a specular (mirror-like) target which has surfaces that are large, planar, and have minor irregularities that are small in comparison to the wavelength of the untrasound being used. In the specular case, the usual laws of reflection and refraction occur at the interface, and the ultrasound waves reflected back to the transducer are in phase. However, even the images of large specular targets, such as the aorta, may have gaps at times. This is due to the waves being reflected from two different levels in the structure, either because of layers in the structure or because of its curvature, and the waves arriving at the transducer out of phase and cancelling. This produces a hole or "dropout" in the image.
The second interaction involves an encounter of the ultrasonic waves with small discrete targets, such as cellular structure, which are smaller than one-half wavelength of the ultrasonic wave. When coherent waves encounter the small targets, which are referred to as Rayleigh scatterers, the ultrasonic waves are scattered or dispersed in all directions. The spherical wavefronts arriving at the transducer from the individual scatterers may add or cancel at any particular time, thereby producing a very fine textured salt-and-pepper interference pattern which is superimposed on the image. This pattern, which is commonly referred to as acoustic speckle, constitutes noise which may have an intensity equal to or higher than the signal of interest. Acoustic speckle is present to some degree at every point in the ultrasonic image. Accordingly, the grey scale level at a point in the image is not necessarily a true representation of the reflected amplitude from a corresponding position in the sample volume. Acoustic speckle blurs the edges of specular targets and impairs the resolution of the resulting image. Because of acoustic speckle and dropouts caused by phase cancellation from specular targets, as described above, the ultrasonic image displayed is not a true representation of the structures in the two-dimensional echo beam.
Although various techniques are known for reducing speckle in images, known techniques have not proved to be satisfactory. One technique which is commonly employed in ultrasonic imaging systems is low pass filtration. Since speckle has a fine texture, and accordingly a high spatial frequency content, this technique may achieve some reduction in speckle. However, the average speckle size is roughly equal to the resolution cell of the ultrasonic imaging system, and low pass filtering causes degradation in resolution. Another technique which has been used for reducing speckle in ultrasonic as well as other types of imaging systems is to process the image using gradient operators or masks to enhance edges in the image. Gradient operators perform a discrete differentiation at each pixel in the image looking for the rate of change in grey level in order to provide a peak intensity in the image at the maximum slope of the rising grey level. Although such image processing techniques do enhance the image to some extent, they do not produce the desired enhancement of selective structures of interest, i.e., the specular reflecting surfaces. Moreover, the processing required to use some gradient operators is difficult to perform in real time and adds a significant amount of complexity and expense to the system.
It is desirable to provide methods and apparatus which operate in real time to reduce noise and afford image enhancement of selective structures and that avoid the foregoing and other disadvantages of known image enhancement techniques, and it is to this end that the present invention is directed.